142 - The Cell 



proton and no neutrons in the center (Fig. 

 8-1). Deuterium, or heavy hydrogen (H 2 ), in 

 contrast, has a neutron in addition to the 

 proton; whereas tritium (H 3 ) has two neu- 

 trons, as is shown in Figure 8-1. But since the 

 planetary system of the atom and its isotopes 

 is identical, the chemical behavior is the 

 same. Consequently, if heavy hydrogen (H-), 

 or radiocarbon (C 14 ), or any other isotope is 

 built into the structure of an organic com- 

 pound, the metabolic fate of the compound 

 should not be changed; this prediction has 

 been substantiated in a great many experi- 

 ments, with very rare, not entirely unequivo- 

 cal, exceptions. 



Isotopes are most valuable for tagging and 

 tracing molecules as they pass through the 

 vortex of metabolic change within the cell. 

 The isotopic compound and its derivatives 

 can be identified and localized by a variety of 

 physical methods. Some isotopes, such as H :! , 

 C 14 , and P 32 (Table 8-1), are unstable and 

 these emit radiation as the atomic centers dis- 

 integrate. Such isotopes and their derivatives, 

 therefore, can be localized when a section of 

 the cell containing them is brought into con- 

 tact with a sensitive photographic plate, as 

 was mentioned previously (p. 21); or they 



may be followed by means of radiation 

 meters, such as a Geiger counter. Stable, non- 

 radioactive isotopes, such as O 18 or N 15 

 (Table 8-1), can also be traced by means of 

 a mass spectrometer, owing to the fact that 

 the atomic mass of the isotope differs from 

 that of the common element. Regardless of 

 detail, however, it is gratifying to know that 

 part of the research in the field of atomic 

 energy has been diverted into significant 

 peace-time channels. 



COMMON 

 HYDROGEN 



(H 1 ) 



HEAVY 



HYDROGEN 



DEUTERIUM 



(H 2 , ORD) 



RADIOACTIVE 



HYDROGEN 



TRITIUM 



(H 3 , ORT) 



Fig. 8-1. An atom and its isotopes possess identical 

 planetary electron systems. Therefore they display an 

 identical chemical behavior. Differences in the atomic 

 centers, however, permit the element and its isotopes 

 to be distinguished on the basis of weight, radioac- 

 tivity, or other physical properties. Symbols: — , elec- 

 tron; +, proton; ±, neutron. 



Table 8-1— Isotopes Useful as Metabolic Tracers 



Symbol 



Half-Life * 

 {if radioactive) 



Principal Field of Use 



Research on water exchanges; general metabolism 

 Localization of metabolic activity 

 General metabolism 



Metabolism of proteins, nucleic acids, etc. 

 Biological oxidations, photosynthesis, etc. 

 Metabolism of ATP, nucleic acids, carbohydrates, etc. 

 Coenzyme A activity; protein metabolism 

 Activity of cytochromes, hemoglobin, etc. 

 Functioning of thyroid gland and hormone 

 Calcification reactions; ionic activities of calcium 

 Active transport mechanisms 

 Cell permeability, etc. 



* As the name implies, the half-life of a radioactive isotope specifies the time required for half of the 

 radioactivity of a given sample to be dissipated. 



